FIG. 1 depicts a schematic diagram of a portion of a wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a wireless telecommunications system is a wireless switching center ("WSC"), which also may be known as a mobile switching center or mobile telephone switching office. Typically, a wireless switching center (e.g., WSC 120) is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic region serviced by the system and to the local and long-distance telephone and data networks (e.g., local-office 130, local-office 138 and toll-office 140). A wireless switching center is responsible for, among other things, establishing and maintaining a call between a first wireless terminal and a second wireless terminal or, alternatively, between a wireless terminal and a wireline terminal (e.g., wireline terminal 150), which is connected to the system via the local and/or long-distance networks.
The geographic region serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called "cells." As depicted in FIG. 1, each cell is schematically represented by a hexagon. In practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with the wireless switching center.
For example, when a user of wireless terminal 101-1 desires to transmit information to a user of wireless terminal 101-2, wireless terminal 101-1 transmits a data message bearing the user's information to base station 103-1. The data message is then relayed by base station 103-1 to wireless switching center 120 via wireline 102-1. Because wireless terminal 101-2 is in the cell serviced by base station 103-1, wireless switching center 120 returns the data message back to base station 103-1, which relays it to wireless terminal 101-2.
Each wireless terminal and each base station comprises a timing signal that it uses for timing its communications with the other. Typically, the timing signal does not provide the exact time (e.g., 3:18 A.M.), but is a waveform with a constant frequency that establishes a cadence for the wireless terminals and base stations to follow.
The ease with which a wireless terminal and a base station can communicate is dependent on the degree to which the timing signal in the wireless terminal and the timing signal in the base station are synchronized. In other words, if the timing signal in the wireless terminal and the timing signal in the base station are not synchronized, communication between the wireless terminal and the base station may be difficult or impossible.
The degree of synchronization of two or more timing signals is not definable by a single parameter. Instead, the degree of synchronization is defined by two parameters: (1) frequency, and (2) phase. To illustrate the relationship and meaning of these two parameters, FIGS. 2 through 4 depicts graphs of pairs illustrative timing signals.
FIG. 2 depicts a graph of two timing signals that are not synchronized because they have different frequencies. In contrast, FIG. 3 depicts a graph of two timing signals that have the same frequency, but are still not synchronized because they have different phases. And finally, FIG. 4 depicts a graph of two timing signals that are synchronized because they have the same frequency and the same phase. In general, for a wireless terminal and a base station to be able to communicate, the timing signal in the wireless terminal and the timing signal in the base station must have the same frequency and nearly the same phase.
It is well known in the prior art how to synchronize a timing signal in a wireless terminal and a timing signal in a base station. In accordance with one technique, the base station transmits its timing signal to the wireless terminal. Periodically or sporadically or continually, the wireless terminal uses the timing signal from the base station to synchronize its own timing signal. Because the base station directs the wireless terminal to synchronize its timing signal to that of the base station, but the base station does not synchronize its timing signal to that of the wireless terminal, the relationship of the base station and the wireless terminal is asymmetric. In particular, the base station acts like a master and the wireless terminal acts like a slave.
There are occasions when a wireless terminal needs to communicate with two or more base stations simultaneously or in relatively short succession. In this case, the timing signal in the wireless terminal is advantageously synchronized with the timing signals in all of the base stations with which it communicates. By implication, this requires that the timing signals in all of the base stations be synchronized with each other. In other words, when a wireless terminal needs to communicate with two or more base stations, the timing signal in the wireless terminal needs to be synchronized with the timing signals in all of the base stations, and all of the base stations' timing signals need to be synchronized with each other. To accomplish this, the base stations synchronize their timing signals with each other and the wireless terminal synchronizes its timing signal with that of one of the base stations.
There are two techniques in the prior art for synchronizing the timing signals of multiple base stations.
In accordance with the first technique, each base station comprises an independent but highly-accurate timing source, such as a cesium clock whose rate of vibration is well-known and very stable under a wide range of environmental conditions. This technique is advantageous because it effectively ensures that each base station's timing signals are synchronized in frequency. This technique is disadvantageous, however, because the independence of the timing sources does nothing to synchronize the timing signals in phase. Therefore, this technique is bound to produce timing signals that are synchronized in frequency, but not phase, such as those shown in FIG. 3.
In accordance with the second technique, all of the base stations derive their timing signals from a reference timing signal that is transmitted from a single timing source. Typically, the timing source is located in a wireless switching center and the reference timing signal is transmitted to each base station via the wireline associated with that base station. Like the first technique, the second technique is advantageous because it effectively ensures that each base station's timing signals are synchronized in frequency. Also like the first technique, the second technique is disadvantageous because the base stations' timing signals are not synchronized in phase.
The reason has to do with geography. Because not all of the base stations are equidistant from the common master timing signal, the reference timing signal must traverse a different distance from the timing source to each base station. And because the reference timing signal propagates from the timing source to each base station at the same velocity, the reference timing signal arrives at each base station at a slightly different time. The arrival of the timing signal at each base station at a slightly different time exhibits itself as a phase disparity in the respective timing signals at the base stations.
Therefore, the need exists for a technique for synchronizing the timing signals in the base stations of a wireless telecommunications system, in both frequency and phase, without some of the costs and disadvantages of techniques in the prior art.